Method and system for television communication



y 9, 1939- A. B. DU MONT 2,157,749

METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Filed Jan. 11, 1938 3 Sheets-Sheet l FRAME DEFLECTION L.\NE

DEFLECTION INVENTOR May 9, 1939.

A. B. DU MONT METHOD AND SYSTEM FOR TELEVISION COMMUNICATION Filed Jan. 11, 1938 S Sheets-Sheet 2 ZOCbMJkMD ZOEIQUAuMO m 26in A. B. DU MONT METHOD AND SYSTEM FOR TELEVISION COMMUNICATION 3 Sheets-Sheet 3 Filed Jan. 11, 1925s mow Q ZOCbMJh m D .N Z J ZOE-UN TED Patented May 9, 1939 UNITED METHOD AND SYSTEM FOR- TELEVISION COMMIUNICATION Allen B. Du Mont, Upper Montclair, N. J., as-

signor to Allen B. Du Mont Laboratories, Inc., Passaic, N. 3., a corporation of Delaware Application January 11, 1938, Serial No. 184,407

1'7 Claims.

My invention relates to improvements in methods and systems for television communication.

In the various television systems used heretofore and in which the scanning devices have been in the form of cathode-ray tubes, it has been necessary for two-way communication to employ at each station separate tubes for pick-up and viewing. That is, at each station in these systems, a cathode-ray tube having a photosensitive screen is used to develop the picture signals for transmission, and a cathode-ray tube having a fluorescent screen is used to produce an image of the view being transmitted from the distant station.

One of the objects of my invention is the provision of an improved system for two-way television communication, in which a single cathoderay tube at each station serves both as a pick-up device to develop picture signals for transmission to the distant station and as a receiver or viewing device to produce an image of the view being transmitted from the distant station, thereby greatly simplifying and reducing the cost of the apparatus over that of the prior systems referred to in which separate pick-up and viewing tubes must be used at each station.

Another object of my invention is the provision of an improved television system in which the single scanning device at each station operates both as a pick-up device and as a viewing device and in such manner as to give simultaneous two-way communication between the two stations.

Other objects and advantages will hereinafter appear.

For the purpose of illustrating my invention, several embodiments thereof are shown in the drawings, in which Figure 1 is a simplified, diagrammatic, plan view of a two-way television system constructed and operating in accordance with my invention;

Fig. 2 is a simplified, diagrammatic, side-elevational view, partly in perspective, of another twoway television system constructed and operating in accordance with my invention;

Fig. 3 is a simplified, diagrammatic, side-elevational view of still another two-way television system constructed and operating in accordance with my invention;

Fig. 4 is an enlarged, fragmentary, sectional view showing the screen construction in Fig. 3;

Fig. 4a is a view similar to Fig. 4, showing a modification; and

Fig. 5 is a simplified, diagrammatic view, partly in perspective, showing a modification of the system in Fig. 3 to give simultaneous two-way communication between two stations.

With reference to Fig. 1, the numerals l0 and II designate cathode-ray tubes disposed respectively at the stations A and B. Since these tubes are similar in construction and manner of operation, only one will be described in detail.

The tube ||l comprises a. photosensitive screen Pa and a fluorescent screen Fa, disposed on opposite sides respectively of the tube axis, and means in the form of an electron gun for developing a ray l3 of electrons focused on and directed at either of the screens. The electron gun consists of a heater M, a cathode IS, a modulating electrode or control grid l6, and anodes ll, I8 and I9.

For the purpose of causing the ray l3 to scan either of the screens Pa. and Fa, a suitable sweep or deflecting circuit 20, which is common to the two tubes as shown and which operates at a suitable line frequency, is connected across the plates 2|. Also, a suitable sweep or deflecting circuit 22, which is common to the two tubes as shown and which operates at a suitable frame frequency, is connected across the plates 23.

In transmitting from station A to station B, a light image of the view or object 24 for transmission is projected by a suitable lens system 25 onto the photosensitive surface of the screen P9. of the tube Ill. Also, the electron ray I3 is made to scan only the screen Pa, whereas the electron ray |3b in tube II is made to scan only the fluorescent surface of the screen Fb in a manner hereinafter explained. Picture signals, accordingly developed in the well known manner by the tube II], are supplied to an amplifier 26 through a lead 2'! common to and coming from the screens Pa and Fa. The amplified picture signals are supplied through a line 28 and the closed switch 29 to the modulating electrode is of tube whereby the intensity of the electron ray |3h is modulated to produce on the fluorescent screen Fb a visible image of the light image on the photosensitive screen Pa, since the two electron rays I3 and |3b are scanning the respective screens synchronously and in the same manner. The image so produced on the fluorescent screen Fb is seen from a point 30.

The picture signals developed by the tube |0 may be taken either from the screen Pa by the lead 21, or from a collector electrode 3| by a lead 32, depending upon the type of photosensitive screen used. That is, when a mosaic, photosensitive surface is used for the screen Pa, the collector electrode 3| is grounded and the picture signals taken from the screen Pa by the lead 21. When a secondary-emissive surface is used for the screen Pa, the latter is grounded and the picture signals are taken from the electrode 3| by the lead 32 and fed to the amplifier 26, the electrode 3| preferably being several hundred volts positive with respect to the screen Pal In transmitting from station B to station A, the switch 29 is opened andthe corresponding switch 29a at station A is closed. A light image of the view or object 24b at the station B is projected by a suitable lens system 25b onto the photosensitive screen Pb. In this case, also, the electron ray I31 is made to scan only the photosensitive screen Pb, and the electron ray I3 is made to scan only the fluorescent screen Fa- The operating action is the same as before. That is, the picture signals developed by the tube II are supplied to the amplifler 26b through the lead 21b, and the amplified picture signals are supplied through the line 28b and the closed switch 293. to the modulating electrode N5 of the tube In. Accordingly, on the fluorescent screen F0. at station A is produced a visible image, seen from the point 303,, of the light image on the photosensitive screen Pb at station B.

The shifting of the electron rays 83 and l3b to cause them to scan the photosensitive and fluorescent screens selectively, is accomplished by changing the positioning voltages applied to the deflecting plates for deflection of the rays at the line frequency. This may be done in a well known manner by changing a value of usual position-control resistance by means of a switch 33, and making the respective polarities such that the electron rays shift in the proper directions. That is, with the switch 33 open, for example, the electron rays l3 and l3b are caused to scan only the screens Pa and Fb, respectively, whereas with this switch closed, the electron rays I3 and I311 are caused to scan only the screens Fe. and Pb, respectively. It will be understood that instead of positioning the rays electrostatically, this may be done by means of a deflecting coil and a local source of D. C. potential.

The photosensitive and fluorescent surfaces have been shown as being formed on one piece of metal, with the common leads 2i and 2%. In some cases, however, it may be advisable to have the photosensitive and fluorescent surfaces on separate plates with separate leads, or to mount the photosensitive surface and the fluorescent surface on separate sheets of mica or glass.

In the drawings, suitable valuesof voltage are designated for the control electrode is and the anodes ll, l8 and is.

In the system shown in Fig. 2, the construction of the electron tubes 34% and 35 at the respective stations C and D is such as to provide for simultaneous, two-way communication between the stations. That is, each tube can transmit and receive at the same time.

In Fig. 2, the construction of the electron guns, and the sweep or deflecting circuits and connections for developing the electron rays '30 and I3 1 to cause these rays to scan the respective screen structures, are the same as in Fig. 1. However, in Fig. 2 the shape of the bulb of the tubes and the disposition of the photosensitive and fluorescent.

screens are different, as shown.

In Fig. 2, the photosensitive screens Po and Pd are supported alongside the fluorescent screens Fe and Fe, respectively, with a slight separation and with separate leads from each screen. Also, the relative position of the two screens in one tube is the opposite of the relative position of the two screens in the other tube.

In the operation of this system, light images of the views or objects 240 and 24s at the two stations are projected simultaneously onto the respective screen structures.

Picture signals developed by the tube 34 are taken from the photosensitive screen Pc by the connection 210 and supplied to the amplifier 26c. The amplified picture signals are supplied through the transmission line 28c to the modulating electrode ltd of the tube 35.

Picture signals developed by the tube 35 are taken from the photosensitive screen Pd by the connection 21d and supplied to the amplifier 26a. The amplified picture signals are supplied through the transmission line 28a to the modulating electrode I66 of the tube 34.

It will be assumed that the electron rays are deflected synchronously, and horizontally at a suitable line frequency, and vertically at a suitable frame frequency. On this account, and because of the opposite relation of the two kinds of screens at the two stations, it will be seen that each tube is caused to operate alternately to transmit picture signals to the other station and to reproduce a visible image in accordance with picture signals received from the other station, and that when one tube is functioning as a receiver the other tube is functioning as a transmitter.

For example, when the electron ray |3d is difeeding picture signals to the video amplifier 26.

As the ray l3c scans the photosensitive screen F0 from point 3 to point 6, the ray l3d will scan the fluorescent screen Fa from point 3 to point 4 and its intensity will be modulated according to the picture signals developed during this time. cycle of operation is repeated each time the electron rays scan the screen structures from left to right. The frame frequency is made sufficiently high so that no flicker appears in the picture or image produced on the fluorescent screens.

This

The same results as above can be obtained by I first scanning screens Fe and Pd completely, and next scanning screens Fe and Fe completely. This can be done by deflecting the electron rays vertically at the line frequency and horizontally at the frame frequency.

In either method of scanning, the amplitude of one scanning voltage is twice that of the other, assuming a picture aspect ratio of 1:1.

The leads from the fluorescent screens Fe and Fe are provided so that if necessary it will be possible to bias these screens so that var ing secondary emission from them will not couple into the photoelectric plate circuits and cause distortion.

In the system shown in Fig. 3, the same screen is utilized for pick-up and viewing, so that each tube can transmit and receive at the same time. In this system, therefore, there can be simultaneous, two-way communication between the stations E and F, as in Fig. 2.

In Fig. 3, the construction of the electron guns, and the sweep or deflecting circuits and connections for developing the electron rays |3e and I3: to cause these rays to scan the respective screen structures, are the same as in Figs. 1 and 2, the shape of the bulbs being more like that in Fig. 2.

The principal difference in the construction of the cathode-ray tubes 35 and 31 in Fig 3, and

those in Figs. 1 and 2, resides in the construction and characteristics of the screens PeFe and PrFs. That is, the entire effective area of each of these screens is both photosensitive and fluorescent, so that during each field or frame, the

electron rays scan a surface which, at every elemental area thereof, is both photosensitive and fluorescent.

One method of making the screens is illustrated in Fig. 4. The construction comprises a metal back or supporting plate 38 and a mica plate 39. A viscous insulating binder is applied to the mica plate 39 through a screen of relatively fine mesh, so that at elemental, uniformly spaced areas over the mica surface there is some of the binder material. Crystals of the fluorescent material 4| are next dusted onto the binder material at the segregated elemental areas. Silver is now sputtered onto the fluorescent surfaces and the ordinary process for caseium cells carried out. This consists in properly oxidizing the silver, flashing the cascium pellet and then baking properly. There then results a mosaic surface which is both photosensitive and fluorescent.

Another method of making the screens in Fig. 3 is illustrated in Fig. 4a. In this case, the construction also comprises a metal back or supporting plate 383. and a mica plate 39a. Minute spots 5 of silverare by the well known halftone process, printed on the mica surface, these spots being uniformly spaced over the surface and adjacent spots being very close to each other. Also printed on the mica surface by the same process, and with a fluorescent ink, for example. are minute, fluorescent spots 5. In the second printing operation, however, the placement is such that the minute fluorescent spots 6 are disposed in the spaces between the minute, silver spots 5. The silver spots 5 are now photosensitized in the same, usual manner as the minute, silver areas in Fig. 4. If desirable, the minute, fluorescent spots 6 may be printed on the mica surface first.

From the foregoing it will be seen that in the construction in Fig. 4, each efiective elemental area or spot of the screen surface is both photosensitive and fluorescent, whereby during the scanning of each spot by the electron ray, the tube can function either as a transmitter or as a receiver. In Fig. 411, however, the alternate effective elemental areas or spots of the screen surface are respectively only photosensitive and fluorescent, whereby the tube can function either as a transmitter or as a receiver.

In this system, there is a common video amplifier 42, and a double pole, double throw switch 43 is used to connect the amplifier for use with either of the tubes 36 and 31.

In transmitting from station F to station E, the switch 43 is placed in position x, and a light image of the view or object 24f at station F is projected onto the screen PrFr. Picture signals developed by the tube 3! are supplied to the input side of amplifier 42 by the lead 27: and. through switch 43. From the output of, the video amplifier, the amplified signals pass through the switch 43.and connection 28f to the control electrode lfie of tube 36, and modulate the intensity of ray 3e in accordance with variations in the picture signals. Since both electron rays are scanning the respective screens synchronously and in the same manner, there will be produced on the fluorescent screen of tube 36 a visible image of the object 24f.

In transmitting from station E to station F, the switch 43 is placed in position y, and a light image of the view or object 248 at station E is projected onto the screen PeFe. Picture signals developed by the tube 36 are supplied to the input side of amplifier 42 by the lead 212 and and through switch 433. From the output of the video amplifier, the amplified signals pass through the switch 43 and connection 28c to the control electrode I61 of tube 31, and modulate the intensity of ray l3: in accordance with variations in the picture signals. There is then produced on the fluorescent screen PrFr of tube 3'! a visible image of the object 24a.

From the foregoing, it will be seen that if the frame frequency is, for example, 60, and the switch 43 were actuated at a. uniform rate to be in each of the positions :1: and z/ 30 times a second, and in these positions alternately and during a frame period, there will be simultaneous, two-way communication between the stations E and F. That is, as in Fig. 2, each tube then functions alternately as a transmitter and as a receiver and at a rate at least as high as the rate for persistence of vision, and when one tube is functioning as a transmitter the other tube is functioning as a receiver. A form of mechanical switch for this purpose, to replace the switch 43 in Fig. 3 is shown in Fig. 5. This comprises rotary contacts 44 and 45 fixed on a shaft 46 insulated at 41 and 48 and driven at the rate of 30 revolutions per second by a synchronous motor 49. The contact 44 engages the fixed contacts 50 and 5i alternately and each at the rate of 30 times per second. The contact 45 engages the fixed contacts 52 and 53 alternately and each at the rate of 30 times per second. In the position of the switch shown, tube 31 is functioning as a transmitter and tube 36 is functioning as a receiver. That is, picture signals from the screen PrFr are supplied through the line 21f, contacts 50 and 44 and brush 54 to the input side of the video amplifier 42. The amplified picture signals are supplied through the brush 55, contacts 45 and 52 and line 28: to the control electrode I 6e of tube 36. The intensity of the ray l3e is therefore modulated in accordance with the picture signals from tube 31 to produce on the fluorescent screen PeFe a visible image of the object at station F. This action takes place during each half revolution of the shaft 46.

During the other half revolutions of the shaft 46 the reverse condition takes place, as will be seen from the drawings. That is, with the contacts 44 and 45 engaging the contacts 5| and 53, respectively, picture signals from the screen PeFe of tube 36 are supplied, through the line 27, contacts 5i and 44, brush 54, amplifier 42, brush 55, contacts 45 and 53, and line 28c, to the control electrode ISr of the tube 31, thereby producing on screen PrF: an image of the object at station E.

It will be understood that in lieu of the mechanical switch in Fig. 5, an electronic switch may be used for the same purpose.

It will be understood that the electron rays can be deflected electromagnetically in one or both directions, instead of electrostatically, as shown and described.

In the various systems, furthermore. instead of using common sweep or deflecting circuits for the two stations, local sweep circuits may be provided at each station, and synchronizing pulses employed to hold these in step.

Also, the various signals can be transmitted from one station to the other over radio circuits, rather than over a direct wire circuit.

Secondary-emission screens, with fixed patterns, may be used in lieu of the photosensitive screens in Figs. 1 and 2, and. the collector electrodes may be omitted and the picture signals taken from the fluorescent screens.

The system shown in Fig. 1 may be used as a regenerative amplifier by closing switches 29 and 29a and adjusting the gain of the video amplifier at the viewing station so the system does not become overloaded. Assuming that station A is transmitting to station B, it is apparent that there will be a change in secondary emission of the fluorescent screen Fb, and this will cause a varying voltage to be fed into the video amplifier 26! Therefore, the electron ray l3 of tube l0 will be modulated, causing a greater signal output from the photosensitive screen Pa than would otherwise be the case.

It will be understood that various modifications of the different embodiments of my invention described herein, are possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention: v

1. In a cathode-ray tube for television communication, screen structure characterized by the fact that at least part of the effective area thereof. is photosensitive at closely spaced elemental areas to form a mosaic of the photosensitive material and at least part of the eiiective area of said screen structure is fluorescent, means for developing a ray of electrons directed at said structure, and means for deflecting said ray to cause the same to scan the photosensitivefluorescent area directly.

2. In the art of television, the method of operation which consists in utilizing the photosensitive characteristic of screen structure to develop picture signals, utilizing the fluorescent characteristic of screen structure to reproduce a picture, and utilizing the same ray of electrons to scan directly the photosensitive and fluorescent screen structure.

3. In a cathode-ray tube for television communication, screen structure characterized by the fact that at least part of the efiective area thereof is secondary emissive and at least part of the effective area thereof is fluorescent, means for developing a ray of electrons directed at said structure, means for deflecting said ray to cause the same to scan the secondary-emissive and fluorescent areas directly, and a collector electrode for supplying picture signals developed incidental to the scanning action.

4. In a system for television communication between two stations, a cathode-ray tube at one of said stations comprising a tube provided with screen structure and with means for developing a ray of electrons focused on and directed at said structure, means for projecting onto at least part of the efi'ective area of said structure a light image of the view for transmission to the other station, and means for deflecting said ray to cause the same to scan said part of the screen structure to develop picture signals, at least part of the eflective area of said screen structure being fluorescent, said tube being provided with means for modulating the intensity of said ray in accordance with occurring variations in picture signals received at said station, said ray-deflecting means being operable to cause the ray to scan said fluorescent part of said screen structure to produce thereon a visible image.

5. In a cathode-ray tube for television cornmunication, a photosensitive screen, a fluorescent screen, means for developing a ray of electrons focused on and directed at either of said screens, means for projecting onto said photosensitive screen a light image of the view for transmission,

and means for controlling and deflecting said ray .munication, a tube, a photosensitive screen and a fluorescent screen dispmed on opposite sides respectively of the tube axis, means for developing a ray of electrons focused on and directed at either of said screens, means for projecting onto said photosensitive screen a light image of the view for transmission, and means for controlling and deflecting said ray to cause the same to scan said screens selectively, said tube being provided with means for modulating the intensity of said ray in accordance with occurring variations in incoming picture signals.

7. In a system for television communication between two stations, cathode-ray tubes disposed respectively at said stations, each of said tubes being provided with screen structure and with means for developing a ray of electrons focused on and directed at said structure, means associated respectively with said tubes for projecting onto at least part of the eifective area of the screen structure a light image of the view for transmission, means associated respectively with said tubes for deflecting the electron ray to cause the same to scan said part of the screen structure to develop picture signals, at least part of the effective area of the screen structure of each tube being fluorescent, each of said tubes being provided with means for modulating the intensity of the electron ray in accordance with occurring variations in picture signals supplied to the tube, the ray-deflecting means of each tube being operable to cause the electron ray to scan said fluorescent part of said screen structure to produce thereon a visible image, and means for transmitting signals from one of said stations to the other.

8. In a cathode-ray tube for two-way television communication, two screens supported alongside each other, one of said screens being fluorescent, means for projecting onto the other of said screens a light image of the view for transmission, means for developing a ray' of electrons focused on and directed at said screens, means for modulating the intensity of said ray in accordance with occurring variations in incoming picture signals, and means for deflecting said ray to cause the same to scan each of said screens substantially the same number of times during each second.

9. In a cathode-ray tube for two-way television communication, a. photosensitive screen and a fluorescent screen supported alongside each other, means for projecting onto said photosensitive screen a light image of the view,for transmission, means for developing a ray of electrons focused on and directed at said screens, means for modulating the intensity of said ray in accordance with occurring variations in incoming picture signals, and means for deflecting said ray to cause the same to scan each of said screens substantially the same number of times during each second.

10. In a system for two-way television communication between two stations, cathode-ray tubes disposed respectively at said stations, each of said tubes being provided with a photosensitive screen and a fluorescent screen supported alongside each other and with means for developing a ray of electrons focused on and directed at said screens, the relative position of the two screens in one tube being the opposite of the relative position of the two screens in the other tube, means associated respectively with said tubes for projecting onto the photosensitive screens thereof light images of the views for transmission, each of said tubes being provided with means for modulating the intensity of the electron ray in accordance with occurring variations in picture signals supplied to the tube, and means associated respectively with said tubes for deflecting the electron ray to cause the same to scan the photosensitive screen and the fluorescent screen substantially the same number of times during each second.

11. In a cathode-ray tube for television communication, screen structure characterized by the fact that the same efi'ective area thereof is both photosensitive and fluorescent, means for developing a ray of electrons focused on and directed at said area, means for projecting onto said area a light image of the view for transmission, means for modulating the intensity of the electron ray in accordance with occurring variations in picture signals supplied to said tube, and means for deflecting the electron ray to caus the sametoscan said area.

12. In a system for television communication between two stations, cathode-ray tubes disposed respectively at said stations, each of said tubes being provided wtih screen structure characterized by the fact that the same eiiective area thereof is both photosensitive and fluorescent, each of said tubes being provided with means for developing a ray of electrons focused on and directed at the screen, means associated respectively with said tubes for projecting onto the screen light images of the views for transmission, each of said tubes being provided with means for modulating the intensity of the electron ray in accordance with occurring variations in picture signals supplied to the tube, each of said tubes being provided with means for deflecting the electron ray to cause the same to scan the screen, and means for transmitting to either tube the picture signals developed by the other.

13. Ina system for television communication between two stations, cathode-ray tubes disposed respectively at said stations, each of said tubes being provided with screen structure and with means for developing a ray of electrons focused on and directed at said structure, means associated respectively with said tubes for projecting onto at least part of the effective area of the screen structure a light image of the view for transmission, at least part of the effective area of the screen structure of each tube being fluorescent, each of said tubes being provided with means for modulatingthe intensity of the electron ray in accordance with occurring variations in picture signals supplied to the tube, a sweep circuit common to the cathode-ray tubes at both form of a cathode-ray tube, a single scanning device at the second station and being in the form of a cathode-ray tube, each of said tubes being provided with screen structure and with means for developing a ray of electrons focused on and directed at said structure, means associated respectively with said tubes for projecting onto at least part of the efiective area of the screen structure a light image of the view for transmission, at least part of the efiective area of the screen structure of each tube being fluorescent, each of said tubes being provided with means for modulating the intensity of the electron ray in accordance with occurring variations in picture signals supplied to the tube, the cathode-ray tubes at each station being provided with means for deflecting the electron ray to cause the same to scan at least part of the effective area of the screen structure to either function to develop picture signals for transmission to the other station or function to produce a visible image in accordance with picture signals received from said other station, and means for causing each of said tubes to perform both of said functions at a rate at least as high as the rate for persistence of vision.

15. In a system for two-way television communication between two stations, apparatus at the first station for either transmitting picture signals to the second station or producing a visible image in accordance with picture Signals received from the second station, said apparatus comprising a single scanning device, and apparatus at the second station for either transmitting picture signals to the first station or producing a visible image in accordance with picture signals received from the first station, said secondnamed apparatus comprising a single scanning device, each of said devices being in the form of a cathode-ray tube'comprising screen structure characterized by the fact that at least part of the effective area thereof is photosensitive and at least part of the effective area thereof is fluorescent, means for developing a ray of electrons directed at said structure, means for deflecting said ray to cause the same to scan said photosensitive-fluorescent screen structure, "and means for modulating the intensity of said ray in accordance with occurring variations in incoming picture signals.

16. In a cathode-ray tube for two-way television communication, screen structure, means for developing a ray of electrons directed at said structure, and means for deflecting said ray to cause the same to scan the adjacent screen surface, each efiective elemental area of said surface being provided with a. photosensitive and fluorescent spot oi the same elemental dimension whereby during the scanning of each elemental area by said my the tube can flmction either as a transmitter or as a receiver.

1']. In a cathode-ray tube for two-way television communication, screen structure, means for developing a ray of electrons directed at said structure, and means for deflecting said my to cause the same to scan the adjacent screen surface. the alternate effective elemental areas of said surface being respectively only photosensi. tive and only fluorescent whereby during the scanning action said tube can function either as a transmitter or as a receiver.

ALLEN B. DU MONT. 

